MGME1 processes flaps into ligatable nicks in concert with DNA polymerase γ during mtDNA replication

Area of Science:

• Mitochondrial DNA replication and repair
• Enzymology
• Molecular genetics

Background:

• MGME1 is a mitochondrial DNA nuclease that prefers single-stranded DNA (ssDNA).
• Loss-of-function mutations in MGME1 cause mitochondrial diseases characterized by DNA abnormalities.
• The precise biochemical role of MGME1 in DNA replication intermediates remains unclear.

Purpose of the Study:

• To investigate the biochemical function of MGME1 in processing flap intermediates during mitochondrial DNA replication.
• To elucidate the interplay between MGME1 and POLγ in DNA strand ligation and processing.

Main Methods:

• Utilized reconstituted in vitro systems to study MGME1 and POLγ activities.
• Assessed the cleavage of flap intermediates by MGME1.
• Analyzed the processing of MGME1-generated products by POLγ, including its 3'-5' exonuclease activity.

Main Results:

• MGME1 cleaves DNA flaps, facilitating ligation of newly replicated strands in conjunction with POLγ.
• MGME1 generates imprecisely processed products (nicks, gaps, short flaps).
• POLγ converts these products into ligatable nicks via extension or 3'-end excision, utilizing its 3'-5' exonuclease activity.

Conclusions:

• MGME1 and POLγ collaborate to process DNA flap intermediates during replication.
• The 3'-5' exonuclease activity of POLγ is critical for limiting strand displacement and enabling nick ligation.
• Findings impact models of RNA primer removal, OriH processing, and DNA repair mechanisms in mitochondria.